1. Field of the Invention
The present invention relates to a liquid crystal display device having a backlight device, and to an image display method for displaying an image signal while controlling light emission of the backlight device.
2. Description of the Related Art
In a liquid crystal display device displaying an image using a liquid crystal panel, the liquid crystal panel itself does not emit light. Therefore, a backlight device is provided, for example, on the back of the liquid crystal panel. The liquid crystal in the panel is switched between an OFF state and an ON state according to applied voltage. When in the OFF state, the liquid crystal panel interrupts light, while, in the ON state, the liquid crystal panel transmits light. For this reason, the liquid crystal display device drives, as electric shutters, multiple pixels within the liquid crystal panel, by controlling the voltage applied to each of the multiple pixels. An image forms by this control of transmission of light from the backlight through the panel.
A cold cathode tube (CCFL (cold cathode fluorescent lamp)) has heretofore been mainly used as a backlight in a backlight device. When using a CCFL in the backlight device, it is common to keep the CCFL at a certain constant lighting state regardless of the brightness of an image signal to be displayed by the liquid crystal panel.
A large share of power consumption by a conventional liquid crystal display device is for the backlight device. Therefore, a liquid crystal display device has a problem of needing a large power consumption in order to keep the backlight in the constant lighting state. For the purpose of solving this problem, various methods have been proposed wherein a light emitting diode (LED) is used as a backlight. The emission luminance of the LED changes according to the brightness of the image signal.
For examples of the letter, see the description of “T. Shirai, S. Shimizukawa, T. Shiga, and S. Mikoshiba, 44.4: RGB-LED Backlights for LCD-TVs with 0D, 1D, and 2D Adaptive Dimming, 1520 SID 06 DIGEST (Non-patent Document 1, below)” and Japanese Patent Application Laid-open Publications Nos. 2005-258403 (Patent Document 1), 2006-30588 (Patent Document 2) and 2006-145886 (Patent Document 3), which describe a backlight device including multiple LEDs that is divided into multiple regions. The emission luminance of the backlight for each region is controlled according to the brightness of the image signal. In particular, Non-patent Document 1 refers to this technique as “adaptive dimming.”
In the conventional liquid crystal display device described in Non-patent Document 1, the multiple divided regions of the backlight device are each partitioned by a light shielding wall. The emission luminance of each region is controlled entirely independently according to the image signal strength for each respective region. The LEDs vary in brightness and color, device by device, for their principal wavelength. The degree of such variation differs among colors of red (R), green (G) and blue (B). For this reason, when the multiple regions of the backlight device are completely separated from each other, the brightness and color varies among the regions. As a result, this produces the problem that an image displayed on the liquid crystal panel differs from an original image.
The brightness and light emission wavelength of an LED has a temperature dependence. In particular, an R LED emits less amounts of light with an increase in device temperature, and also experiences a large change of wavelength. In addition, the R, G and B devices have different properties in terms of age deterioration. For this reason, the foregoing problem is particularly acute due to change in temperatures of the LED devices and due to age deterioration of the LED devices.
In the configuration wherein the regions are completely separated from each other, it is difficult to determine the locations of adjacent regions of a particular pixel located above a boundary between the adjacent regions. This is because the manufacturing accuracy of the backlight device is far lower than that of the liquid crystal panel. For this reason, the configuration described in Non-patent Document 1 is not very useful.
In addition, as disclosed in non-patent document 1 and in patent documents 1 to 3, power consumption can be reduced by employing a configuration wherein a backlight device is divided into multiple regions, and in which the emission luminance of a backlight for each region is controlled according to the brightness of an image signal. Power consumption, however, is expected to be further reduced.